arXiv subjects

E. Morosan

Publications and source records attributed to E. Morosan.

At least 37 records · Page 2Linked to original sources

arXiv2018

Effects of chemical disorder in the itinerant antiferromagnet Ti$_{1-x}$V$_x$Au

The fragile nature of itinerant magnetism can be exploited using non-thermal parameters to study quantum criticality. The recently discovered quantum critical point (QCP) in the Sc-doped (hole-like doping) itinerant antiferromagnet TiAu (Ti$_{1-x}$Sc$_{x}$Au) raised questions about the effects of the crystal and electronic structures on the overall magnetic behavior. In this study, doping with V (electron-like doping) in Ti$_{1-x}$V$_{x}$Au introduces chemical disorder which suppresses antiferromagnetic order from $T_{\rm N} =$ 36~K for $x = 0$ down to 10 K for $x =$ 0.15, whereupon a solubility limit is reached. Signatures of non-Fermi-liquid behavior are observed in transport and specific heat measurements similar to Ti$_{1-x}$Sc$_{x}$Au, even though Ti$_{1-x}$V$_{x}$Au is far from a QCP for the accessible compositions $x \leq 0.15$.

cond-mat.str-el↗

arXiv2018

Anomalous metamagnetism in the low carrier density Kondo lattice YbRh3Si7

We report complex metamagnetic transitions in single crystals of the new low carrier Kondo antiferromagnet YbRh3Si7. Electrical transport, magnetization, and specific heat measurements reveal antiferromagnetic order at T_N = 7.5 K. Neutron diffraction measurements show that the magnetic ground state of YbRh3Si7 is a collinear antiferromagnet where the moments are aligned in the ab plane. With such an ordered state, no metamagnetic transitions are expected when a magnetic field is applied along the c axis. It is therefore surprising that high field magnetization, torque, and resistivity measurements with H||c reveal two metamagnetic transitions at mu_0H_1 = 6.7 T and mu_0H_2 = 21 T. When the field is tilted away from the c axis, towards the ab plane, both metamagnetic transitions are shifted to higher fields. The first metamagnetic transition leads to an abrupt increase in the electrical resistivity, while the second transition is accompanied by a dramatic reduction in the electrical resistivity. Thus, the magnetic and electronic degrees of freedom in YbRh3Si7 are strongly coupled. We discuss the origin of the anomalous metamagnetism and conclude that it is related to competition between crystal electric field anisotropy and anisotropic exchange interactions.

cond-mat.str-el↗

arXiv2018

CeIr$_{3}$Ge$_{7}$: a local moment antiferromagnetic metal with extremely low ordering temperature

CeIr$_3$Ge$_7$ is an antiferromagnetic metal with a remarkably low ordering temperature $T_{\rm N}$ = 0.63 K, while most Ce-based magnets order between 2 and 15 K. Thermodynamic and transport properties as a function of magnetic field or pressure do not show signatures of Kondo correlations, interaction competition, or frustration, as had been observed in a few antiferromagnets with comparably low or lower $T_{\rm N}$. The averaged Weiss temperature measured below 10 K is comparable to $T_{\rm N}$ suggesting that the RKKY exchange coupling is very weak in this material. The unusually low $T_{\rm N}$ in CeIr$_3$Ge$_7$ can therefore be attributed to the large Ce-Ce bond length of about 5.7 Å, which is about 1.5 Å larger than in the most Ce-based intermetallic systems.

cond-mat.str-el↗

arXiv2018

Charge density wave behavior and order-disorder in the antiferromagnetic metallic series Eu(Ga_1-xAl_x)_4

The solid solution Eu(Ga_1-xAl_x)_4 was grown in single crystal form to reveal a rich variety of crystallographic, magnetic, and electronic properties that differ from the isostructural end compounds EuGa_4 and EuAl_4, despite the similar covalent radii and electronic configurations of Ga and Al. Here we report the onset of magnetic spin reorientation and metamagnetic transitions for x = 0 - 1 evidenced by magnetization and temperature-dependent specific heat measurements. T_N changes non-monotonously with x, and it reaches a maximum around 20 K for x = 0.50, where the a lattice parameter also shows an extreme (minimum) value. Anomalies in the temperature-dependent resistivity consistent with charge density wave behavior exist for x = 0.50 and 1 only. Density functional theory calculations show increased polarization between the Ga-Al covalent bonds in the x = 0.50 structure compared to the end compounds, such that crystallographic order and chemical pressure are proposed as the causes of the charge density wave behavior.

cond-mat.str-el↗

arXiv2016

Modulated magnetism and anomalous electronic transport in $\rm Ce_3Cu_4As_4O_2$

The complex magnetism and transport properties of tetragonal Ce$_3$Cu$_4$As$_4$O$_2$ were examined through neutron scattering and physical properties measurements on polycrystalline samples. The lamellar structure consists of alternating layers of $\rm CeCu_4As_4$ with a single square Ce lattice and oxygen-linked Ce bi-layer $\rm Ce_2O_2$. Extending along $\bf c$, a tube-like Fermi surface from DFT calculations points to a quasi-two-dimensional electronic system. Peaks in the specific heat at the Neél temperature $T_{N}=24$ $\rm K$, $T_{2}~=~16 $ $\rm K$ and $T_{3}~=~1.9$ $ \rm K$ indicate three magnetic phase transitions or distinct cross-over phenomena. For $T<T_{N}$ neutron diffraction indicates the development of ferromagnetic ab sheets for both Ce sites, with alternating polarization along $\bf{c}$, a wave vector ${\bf k}_{1}={\bf c}^*$. For $T<T_{2}$, quasi-two-dimensional low-energy spin fluctuations with ${\bf k}_{2}=\frac{1}{2}{\bf a}^*$ and polarized perpendicular to ${\bf k}_{2}$ are suppressed. The data are consistent with quasi-two-dimensional antiferromagnetic order in the $\rm CeCu_4As_4$ planes polarized along the ${\bf k}_{2}$ wave vector. $T_{3}$ marks a spin-flop transition where the ${\bf k}_{1}$ staggered magnetization switches to in-plane polarization. While the narrow 4f bands lie deep below the Fermi surface, there are significant transport anomalies associated with the transitions; in particular a substantial reduction in resistivity for $T<T_{N}$. At $T=100$ $ \rm mK$ the ${\bf k}_1$ modulated staggered moment is $0.85~μ_B$, which matches the $0.8~μ_B$ saturation magnetization achieved for H $~=~7$ $ \rm T$ at $T~=~2$ $ \rm K$. From low T Lorentzian fits the correlation length is in excess of 75 Å. We argue the unusual sequence of magnetic transitions results from competing interactions and anisotropies for the two Ce sites.

cond-mat.str-el↗

arXiv2015

Very large magnetoresistance in Fe$_{0.28}$TaS$_{2}$ single crystals

Magnetic moments intercalated into layered transition metal dichalcogenides are an excellent system for investigating the rich physics associated with magnetic ordering in a strongly anisotropic, strong spin-orbit coupling environment. We examine electronic transport and magnetization in Fe$_{0.28}$TaS$_{2}$, a highly anisotropic ferromagnet with a Curie temperature $T_{\mathrm{C}} \sim 68.8~$K. We find anomalous Hall data confirming a dominance of spin-orbit coupling in the magnetotransport properties of this material, and a remarkably large field-perpendicular-to-plane MR exceeding 60% at 2 K, much larger than the typical MR for bulk metals, and comparable to state-of-the-art GMR in thin film heterostructures, and smaller only than CMR in Mn perovskites or high mobility semiconductors. Even within the Fe$_x$TaS$_2$ series, for the current $x$ = 0.28 single crystals the MR is nearly $100\times$ higher than that found previously in the commensurate compound Fe$_{0.25}$TaS$_{2}$. After considering alternatives, we argue that the large MR arises from spin disorder scattering in the strong spin-orbit coupling environment, and suggest that this can be a design principle for materials with large MR.

cond-mat.str-el↗

arXiv2014

Ti$_{1-x}$Au$_x$ Alloys: Hard Biocompatible Metals and Their Possible Applications

The search for new hard materials is often challenging from both theoretical and experimental points of view. Furthermore, using materials for biomedical applications calls for alloys with high biocompatibility which are even more sparse. The Ti$_{1-x}$Au$_x$ ($0.22 \leq x \leq 0.8$) exhibit extreme hardness and strength values, elevated melting temperatures (compared to those of constituent elements), reduced density compared to Au, high malleability, bulk metallicity, high biocompatibility, low wear, reduced friction, potentially high radio opacity, as well as osseointegration. All these properties render the Ti$_{1-x}$Au$_x$ alloys particularly useful for orthopedic, dental, and prosthetic applications, where they could be used as both permanent and temporary components. Additionally, the ability of Ti$_{1-x}$Au$_x$ alloys to adhere to ceramic parts could reduce the weight and cost of these components.

cond-mat.mtrl-sci↗

arXiv2014

Non-Fermi Liquid Behavior Close to a Quantum Critical Point in a Ferromagnetic State without Local Moments

A quantum critical point (QCP) occurs upon chemical doping of the weak itinerant ferromagnet Sc_{3.1}In. Remarkable for a system with no local moments, the QCP is accompanied by non-Fermi liquid (NFL) behavior, manifested in the logarithmic divergence of the specific heat both in the ferro- and the paramagnetic states. Sc_{3.1}In displays critical scaling and NFL behavior in the ferromagnetic state, akin to what had been observed only in f-electron, local moment systems. With doping, critical scaling is observed close to the QCP, as the critical exponents, and delta, gamma and beta have weak composition dependence, with delta nearly twice, and beta almost half of their respective mean-field values. The unusually large paramagnetic moment mu_PM~1.3 mu_B/F.U. is nearly composition-independent. Evidence for strong spin fluctuations, accompanying the QCP at x_c = 0.035 +- 0.005, may be ascribed to the reduced dimensionality of Sc_{3.1}In, associated with the nearly one-dimensional Sc-In chains.

cond-mat.str-el↗

arXiv2014

Strong magnetic coupling in the hexagonal R5Pb3 compounds (R = Gd-Tm)

We have synthesized R5Pb3 (R = Gd-Tm) compounds in polycrystalline form and performed structural analysis, magnetization, and neutron scattering measurements. For all R5Pb3 reported here the Weiss temperatures θW are several times smaller than the ordering temperatures TORD, while the latter are remarkably high (TORD up to 275 K for R = Gd) compared to other known R-M binaries (M = Si, Ge, Sn and Sb). The magnetic order changes from ferromagnetic in R = Gd, Tb to antiferromagnetic in R = Dy-Tm. Below TORD, the magnetization measurements together with neutron powder diffraction show complex magnetic behavior and reveal the existence of up to three additional phase transitions. We believe this to be a result of crystal electric field effects responsible for high magnetocrystalline anisotropy. The R5Pb3 magnetic unit cells for R = Tb-Tm can be described with incommensurate magnetic wave vectors with spin modulation either along the c axis in R = Tb, Er and Tm or within the ab-plane in R = Dy and Ho.

cond-mat.str-el↗

arXiv2014

Novel Itinerant Antiferromagnet TiAu

Itinerant and local moment magnetism have substantively different origins, and require distinct theoretical treatment. A unified theory of magnetism has long been sought after, and remains elusive, mainly due to the limited number of known itinerant magnetic systems. In the case of the two such examples discovered several decades ago, the itinerant ferromagnets ZrZn_2 and Sc_3In, the understanding of their magnetic ground states draws on the existence of 3d electrons subject to strong spin fluctuations. Similarly, in Cr, an elemental itinerant antiferromagnet (IAFM) with a spin density wave (SDW) ground state, its 3d character has been deemed crucial to it being magnetic. Here we report the discovery of the first IAFM compound with no magnetic constituents, TiAu. Antiferromagnetic order occurs below a Neel temperature T_N ~ 36 K, about an order of magnitude smaller than in Cr, rendering the spin fluctuations in TiAu more important at low temperatures. This new IAFM challenges the currently limited understanding of weak itinerant antiferromagnetism, while providing long sought-after insights into the effects of spin fluctuations in itinerant electron systems.

cond-mat.str-el↗

arXiv2014

Topological metal behavior in GeBi2Te4 single crystals

The metallic character of the GeBi2Te4 single crystals is probed using a combination of structural and physical properties measurements, together with density functional theory (DFT) calculations. The structural study shows distorted Ge coordination polyhedra, mainly of the Ge octahedra. This has a major impact on the band structure, resulting in bulk metallic behavior of GeBi2Te4, as indicated by DFT calculations. Such calculations place GeBi2Te4 in a class of a few known non-trivial topological metals, and explains why an observed Dirac point lies below the Fermi energy at about -0.12eV. A topological picture of GeBi2Te4 is confirmed by the observation of surface state modulations by scanning tunneling microscopy (STM).

cond-mat.mtrl-sci↗

arXiv2013

Quasi-two-dimensional non-collinear magnetism in the Mott insulator Sr$_2$F$_2$Fe$_2$OS$_2$

The magnetism of Sr$_2$F$_2$Fe$_2$OS$_2$ was examined through neutron powder diffraction and thermodynamic and transport measurements. Quasi-two-dimensional magnetic order develops below $T_N$ = 106(2) K with an in-plane correlation length exceeding 310 Å and an out-of-plane correlation length of just 17(3) Å. The data are consistent with a two-k structure with $k_1$ = (1/2, 0, 1/2) and $k_2$ = (0, 1/2, 1/2) and an ordered moment of 3.3(1) $μ_B$ oriented along the in-plane components of k. This structure is composed of orthogonal AFM chains intersecting at super-exchange mediating O sites. Density Functional Theory (DFT) also points to this structure and a narrower Fe 3d band than for the iron pnictides from which electronic correlations produce a Mott insulator.

cond-mat.str-el↗

arXiv2012

A topological crystalline insulator (TCI) phase via topological phase transition and crystalline mirror symmetry

A Z2 topological insulator protected by time-reversal symmetry is realized via spin-orbit interaction driven band inversion. For example, the topological phase in the Bi-Sb system is due to an odd number of band inversions. A related spin-orbit system, the (Pb/Sn)Te class, has been known to contain an even number of inversions based on band theory. Here we experimentally investigate the possibility of a mirror symmetry protected topological crystalline insulator phase in the (Pb/Sn)Te class of materials which has been theoretically predicted to exist in its non-alloyed version. Our experimental results show that at a finite-Pb composition above the topological inversion phase transition, the surface exhibits even number of spin-polarized Dirac cone states (as opposed to odd as observed in Bi-Sb alloy or Bi2Se3) revealing mirror protected topological order distinct from that observed in Bi-Sb or Bi2Se3. Our observation of the spin-polarized Dirac surface states in the inverted (Pb/Sn)Te and their absence in the non-inverted compounds related via a topological phase transition (spin-orbit induced band inversion type) provide the experimental groundwork for opening the research on novel topological order in future quantum devices.

cond-mat.mes-hall↗

arXiv2012

Observation of Topological Crystalline Insulator phase in the lead tin chalcogenide Pb1-xSnxTe material class

We perform systematic angle-resolved photoemission spectroscopic measurements on the lead tin telluride Pb1-xSnxTe pseudobinary alloy system. We show that the (001) crystalline surface, which is a crystalline surface symmetric about the (110) mirror planes of the Pb1-xSnxTe crystal, pos- sesses four metallic surface states within its surface Brillouin zone. Our systematic Fermi surface and band topology measurements show that the observed Dirac-like surface states lie on the symmetric momentum-space cuts. We further show that upon going to higher electron binding energies, the surface states' isoenergetic countours in close vicinity of each X point are observed to hybridize with each other, leading to a Fermi surface fractionalization and the Lifshitz transition. In addition, systematic incident photon energy dependent measurements are performed, which enable us to un- ambiguously identify the surface states from the bulk bands. These systematic measurements of the surface and bulk electronic structure on Pb1-xSnxTe, supported by our first principles calculation results, for the first time, show that the Pb1-xSnxTe system belongs to the topological crystalline insulator phase due to the four band inversions at the L points in its Brillouin zone, which has been recently theoretically predicted.

cond-mat.mes-hall↗

arXiv2012

Type-I superconductivity in ScGa3 and LuGa3 single crystals

We present evidence of type-I superconductivity in single crystals of ScGa3 and LuGa3, from magnetization, specific heat, and resistivity measurements: low critical temperatures Tc = 2.1-2.2 K; field-induced secondto first-order phase transition in the specific heat, critical fields less than 240 Oe; and low Ginzburg-Landau coefficients κ approx 0.23 and 0.30 for ScGa3 and LuGa3, respectively, are all traits of a type-I superconducting ground state. These observations render ScGa3 and LuGa3 two of only several type-I superconducting compounds, with most other superconductors being type II (compounds and alloys) or type I (elemental metals and metaloids).

cond-mat.supr-con↗

arXiv2012

Type I Superconductivity in YbSb2 Single Crystals

We present evidence of type I superconductivity in YbSb2 single crystals, from DC and AC magnetization, heat capacity and resistivity measurements. The critical temperature and critical field are determined to be $T_c\approx$ 1.3 K and $H_c\approx$ 55 Oe. A small Ginzburg-Landau parameter κ= 0.05, together with typical magnetization isotherms of type I superconductors, small critical field values, a strong Differential Paramagnetic Effect (DPE) signal, and a field-induced change from second to first order phase transition, confirm the type I nature of the superconductivity in YbSb2. A possible second superconducting state is observed in the radiofrequency (RF) susceptibility measurements, with $T_{c}^{(2)}\approx$ 0.41 K and $H_{c}^{(2)}\approx$ 430 Oe.

cond-mat.supr-con↗

arXiv2011

Anomalous magneto-optical Kerr hysteresis loops in Fe$_{0.25}$TaS$_{2}$

We have performed magneto-optical Kerr spectroscopy measurements on intercalated transition-metal dichalcogenide Fe$_{0.25}$TaS$_{2}$ in the polar Kerr geometry as a function of temperature, magnetic field, and wavelength. The Kerr angle exhibits pronounced peaks at $\sim$775 nm (1.6 eV) and $\sim$515 nm (2.4 eV), which we attribute to spin-dependent interband optical transitions arising from states in the vicinity of the Fermi energy. Below the ferromagnetic transition temperature (165 K), we observe strongly wavelength- and magnetic-field-dependent Kerr signal. At a fixed wavelength, the magnetic-field dependence of the Kerr angle shows a clear hysteresis loop, but its shape sensitively changes with the wavelength. We propose a model that takes into account contributions from domain walls, which allowed us to derive a mathematical expression that successfully fits all the observed hysteresis loops.

cond-mat.str-el↗

arXiv2011

Thermal and electrical transport in the spin density wave antiferromagnet CaFe$_{4}$As$_{3}$

We present here measurements of the thermopower, thermal conductivity, and electrical resistivity of the newly reported compound CaFe4As3. Evidence is presented from specific heat and electrical resistivity measurements that a substantial fraction of the Fermi surface survives the onset of spin density wave (SDW) order at the Neel temperature TN=88 K, and its subsequent commensurate lockin transition at T2=26.4 K. The specific heat below T2 consists of a normal metallic component from the ungapped parts of the Fermi surface, and a Bardeen-Cooper- Schrieffer (BCS) component that represents the SDW gapping of the Fermi surface. A large Kadowaki-Woods ratio is found at low temperatures, showing that the ground state of CaFe4As3 is a strongly interacting Fermi liquid. The thermal conductivity of CaFe4As3 is an order of magnitude smaller than those of conventional metals at all temperatures, due to a strong phonon scattering. The thermoelectric power displays a sign change from positive to negative indicating that a partial gap forms at the Fermi level with the onset of commensurate spin density wave order at T2=26.4 K. The small value of the thermopower and the enhancements of the resistivity due to gap formation and strong quasiparticle interactions offset the low value of the thermal conductivity, yielding only a modest value for the thermoelectric figure of merit Z < 5x10^-6 1/K in CaFe4As3. The results of ab initio electronic structure calculations are reported, confirming that the sign change in the thermopower at T2 is reflected by a sign change in the slope of the density of states at the Fermi level. Values for the quasiparticle renormalization are derived from measurements of the specific heat and thermopower, indicating that as T->0, CaFe4As3 is among the most strongly correlated of the known Fe-based pnictide and chalcogenide systems.

cond-mat.str-el↗